1. Field of the Invention
This invention relates to a method of producing steel superior in strength, toughness and weldability by virtue of having been put through controlled rolling combined with controlled cooling.
Recently, various kinds of high tension steel have been used for many welded constructions such as buildings, pressure vesseles, ship building and pipe lines to satisfy economic requirements and safety in use, accordingly, the demand for high tension steels having good weldability has been steadily increasing.
Steel used for welded constructions is required to have high toughness and superior weldability for the sake of safety and good workability in welding operation in addition to high tensile strength.
2. Description of the Prior Art
It is well known that controlled rolling and quenching and tempering methods applied for making line pipes have heretofor been used for making high tension steel which satisfies the above-mentioned requirements.
However, microstructure obtained by the former method is of two phase structure consisting of ferrite and pearlite, so there exists limitation with respect to the strength and the thickness of the rolled products.
In other words, large amounts of alloying elements have to be added in order that the steel having acicular ferrite or bainite structure can be obtained.
On the other hand, the latter method requires at least a reheating step which gives rise to high production costs and further has limitation due to production capacity.
In view of these drawbacks, a more advanced method of controlled cooling is being developed which enables saving of energy and natural resources, that is, lowering addition of alloying elements.
Steels obtained by this improved method display the features of the steel obtained through both the controlled rolling method (hereinafter referred to as CR method) and the quenching and tempering method (hereinafter referred to as QT method) and are able to provide superior properties with either low addition of alloying elements or even without addition of any special alloying element or elements.
However, those steels manufactured by the conventional methods still have suffered from several drawbacks as mentioned below.
(1) A tempering step is indispensable for a steel which has been subjected to quenching after rolling, so as to restore the ductility and toughness lowered by the quenching.
(2) Since the extent of softening at the heat affected zone (HAZ) of the steel due to welding is remarkably large, it is very difficult, particularly for high yield point steels or high tension steels, to ensure required strength.
(3) Microstructure of the steel in the direction of its thickness is nonuniform and has large extent of hardness variation.
(4) Condition of cooling (temperature of starting and stopping the cooling and the rate of cooling) must be controlled in a very strict manner, thus it is liable to cause undesired variations in the property of the product steel.
Due to these drawbacks, steels manufactured in accordance with the conventional controlled cooling method have been applied only for very restricted uses and also due to the difficulty for making on a mass production scale, they have not been widely used to up to the present.
With an intention to obviate aforesaid defects, the inventors of the present invention carefully studied various factors such as chemical composition of the steel to be used well as a condition of heating, rolling and the manner of cooling the steel.
As a result, the inventors have developed a novel method of making a series of high toughness steels accomplished by combining low temperature heating followed by controlled cooling and have filed two prior patent applications in Japan, their Patent Laid-open Nos. are 131125/80 and 76126/82.
Through further studies and experiments, the inventions have also found an entirely new method of making steels other than those disclosed by the above-mentioned prior inventions.